Automatic gain control (AGC) is used to proportionally adjust the voltage of an input analog signal to provide an output signal within a desired range, e.g., required by downstream signal processing. Amplifiers having AGC employ active components that are in the signal path between the input and output and cause changes in phase with changes in gain, something which cannot be tolerated in some applications, e.g., those employing multiple channels.
Attenuators proportionally reduce the voltage of an input analog signal and can be used with an AGC circuit that senses a resulting output and controls the attenuator so as to keep the output within a desired range. Passive attenuators include only passive elements (e.g., resistors) in the signal path between the signal input and output, thereby avoiding phase change with change in gain. The so called "T-pad" attenuator circuit employs fixed value resistors in the signal path between the input and output and a variable resistance between the signal path and ground. The circuit acts as a voltage divider and can be provided in stages to increase attenuation. The variable resistances have been provided by electromechanical potentiometers and by low speed voltage controlled devices, e.g., junction field effect transistors (JFETs), as described in "FETs As Voltage-Controlled Resistors", Siliconix, Inc., Santa Clara, Calif. 95054 (1986) pp. 7-75 to 7-83.
An application requiring an ultrahigh frequency (UHF), passive attenuator is the receiver of a fiber optic link transmitting frequencies up to 300 MHz and greater.
A known high-frequency attenuator, capable of frequencies up to 15 MHz, is available from Topaz Semiconductor under the CDG4469 trade designation. It employs digital input logic to control eight voltage dividers in series. Each divider has a resistance between the signal path and ground of a different value, and these resistances are selectively switched to ground via respective high-speed, low-capacitance, double diffusion metal-oxide semiconductor (DMOS) field-effect transistor (FET) switches controlled by the input logic. By selectively connecting different combinations of the dividers, 0 to 127.5 dB can be provided in 0.5 dB steps.